The present invention relates to a pulse type transformer intended to be used in a high current pulse type application such as a capacitive discharge type circuit, and in particular, to the number of primary and secondary windings and their physical relation and proximity to each other in a transformer of a coreless design.
The present invention also relates to a pulse type transformer intended to be used in a high current pulse type application such as a capacitive discharge type circuit, and in particular, to the cross-sectional area of a transformer""s core along the core""s magnetic path as related to the location of the primary and secondary windings.
Transformers are electrical devices used to supply power or a signal from an AC source to an AC load. They may also be used to electrically isolate the load from the supply. Transformers consist of one or more input or primary windings along with one or more output or secondary windings which are electrically coupled together through a magnetic material and through the air. The relationship between the amount of power provided from the secondary windings in reference to the amount of power provided to the primary windings is referred to as the coupling coefficient. A change in the coupling coefficient can be provided through a change in core material, core size, winding material, winding size, proximity of the primary windings to the core, proximity of the core to the secondary windings, proximity of the primary windings to the secondary windings, along with change to several other parameters. Although transformers are typically referred to as AC devices, pulsing DC into a transformer""s primary winding causes a change in the magnetic field allowing the transformer to function.
Transformers can be classified into two groups, AC, and pulse type. The AC type transformer can be subdivided into transformers without magnetic cores and transformers with magnetic cores.
The AC type transformer provided without a magnetic core is typically used in high frequency applications such as radio and TV circuits. AC transformers used in high frequency applications are not generally used for power but are used to pass low power signals from the primary to secondary winding. In medium and low frequency applications, coreless transformers are not used because the coupling coefficient is low providing minimal power or signal from the secondary winding. The AC transformers used in high frequency applications are also generally constructed with a single primary winding and single secondary winding that are placed in close proximity to each other such that the energy in the primary winding is radiated to the secondary winding similar to a radio wave. In addition, several AC type transformers used in radio and TV circuits have a very low material cost to manufacturer since they consist primarily of wire and a paper tube or insulator to wind the wire around. This lower cost coreless design would be advantageous if applied to lower frequency, higher power applications where product cost is an important part of the products specification provided the coupling coefficient were increased to meet product specifications.
The AC type transformer provided with a core is typically used in medium and low frequency applications such as power supply and audio circuits. The construction method for these transformers typically consists of winding the primary winding and secondary winding around an insulator or on a bobbin such that the primary and secondary windings are wound around a magnetic core material and such that the primary windings are separated from the secondary windings either by insulation or by being placed on separate locations along the magnetic path of the magnetic material.
The AC transformers used in these medium and low frequency applications may also be provided with more than one primary winding and may also be provided with more than one secondary winding. In AC type transformers with magnetic cores where multiple primary and multiple secondary windings are used, the primary windings are typically kept separate from the secondary windings to provide isolation from primary to secondary and to reduce manufacturing costs. However; in some cases where large transformers are used for applications such as power distribution by utility companies, the primary and secondary windings are wound with alternating primary and secondary windings to decrease the distance between the primary windings and secondary windings in order to provide a slight increase in the coupling coefficient. The transformer construction for these large AC type transformers with alternating primary and secondary windings always includes a magnetic core.
The magnetic materials used in the AC transformer construction vary in shape, size and material. In all cases, the magnetic core must have a complete path (including gaps through air) for the magnetic field in order for the transformer to function efficiently. To minimize resistances within the magnetic field, the air gaps are reduced to a minimum, and core designs are such that the cross-sectional area of the core along the magnetic path is as close to constant as possible. Changes in cross-sectional area along the magnetic path such that the cross-sectional area is not constant, results in a decrease in the coupling coefficient. Although a loss is created by an inconsistent cross-sectional core area, some transformers change the cross-sectional area within a high voltage winding to provide additional distance between the core and high voltage windings. These changes to the cross-sectional area of the core area are limited to within a high voltage winding and do not increase the cross-sectional area of the core within the windings as compared to the cross-sectional area of the core outside the windings.
The second general classification of transformer is the pulse type transformer. Pulse type transformers are similar to the AC transformer provided with a core in construction because they may be provided with multiple primary or secondary windings. They are also similar because they are provided with a magnetic core where the cross-sectional area of the core is constant along the magnetic path with the exception of within a high voltage winding as in the AC transformer.
The pulse type transformer differs from the AC transformer. While the AC transformer is subjected to sinusoidal AC, the pulse type transformer is subjected to repetitive DC pulses, off-periods between pulses, along with the rise-time and fall-time associated with the pulse. As the off-period is increased in the pulse type transformer, the energy in the DC pulse can also be increased without causing over-heating. This increase in pulse energy requires the pulse type transformer be able to withstand high voltages and high peak currents compared to the AC type transformer.
When the current in the primary winding is significantly increased in applications, (such as capacitive discharge circuits typically used in strobe circuits, electric fence controllers, and high performance ignition systems for automobile, motorcycle, or marine engines), the transformer""s coils and magnetic core material behave differently in two ways. First, the primary and secondary windings behave differently in that their proximity to each other provides a significant effect on the coupling coefficient. Second, the cross-sectional area of the core within the primary winding and secondary winding have a more significant effect on the coupling coefficient than the cross-sectional area of the core that is not within the primary winding or secondary winding.
In accordance with the present invention there are several embodiments of a novel high current, low duty cycle, pulse type transformer which include a method of increasing the coupling coefficient through multiple primary and secondary windings and their proximity to each other in a transformer of a coreless design, and which include a method of increasing the coupling coefficient through change in the cross-sectional area of a magnetic core along the magnetic path of the core in a transformer designed with a magnetic core.
The invention utilizes a coreless pulse type transformer with multiple primary windings and multiple secondary windings physically arranged such that groups of secondary windings are separated by groups of primary windings. The coil construction method of placing groups of primary windings between groups of secondary windings in close proximity to each other significantly increases the coupling coefficient for transformers subjected to high supply current, low duty cycle transformers used in applications such as capacitive discharge circuits typically used in strobe circuits, electric fence controllers, and high performance ignition systems for automobile, motorcycle, or marine engines. Although the coreless design requires higher peak supply currents, costs associated with increases in the peak supply current necessary to drive such a coreless transformer may be offset by the reduction in the coreless transformer cost compared to a transformer designed with a magnetic core.
The invention also utilizes a pulse type transformer with one or more primary windings, one or more secondary windings, and a magnetic core where the cross-sectional area of the magnetic core changes along the magnetic path of the magnetic core such that the cross-sectional area is significantly increased within the primary windings and/or secondary windings. Given a volume of core material with a constant cross-sectional area along the magnetic path, a transformer will have a given coupling coefficient. Increasing the cross-sectional area of the core area within the primary and/or secondary windings provides an increase in the coupling coefficient by a given value. Decreasing the cross-sectional area of the same core outside the primary and secondary windings by the same amount as the increase within the windings decreases the coupling coefficient by a value that is less than the increase in coupling coefficient associated with the increase in cross-sectional area within the windings. The result is an increase in the coupling coefficient for the same volume and cost of core material for a pulse type transformer that subjected to high supply current, low duty cycle applications such as capacitive discharge circuits previously mentioned like those used in strobe circuits, electric fence controllers, and high performance ignition systems for automobile, motorcycle, or marine engines.